Many communication signal processing systems and techniques are based on assumptions regarding noise and interference. In Orthogonal Frequency Division Multiplexing (OFDM) systems, for example, received signal processing typically assumes Additive White Gaussian Noise (AWGN) that is constant across all sub-carriers or tones. In the presence of dispersive channels for both a desired communication signal and one or more interference signals, however, interference tends not to be constant across channels. This phenomenon is often referred to as “coloured” interference. Interference caused by a single interference source can be coloured in the frequency domain, due to multi-path effects, for example.
Conventional signal processing operations such as decoding and demodulation of received signals are affected when these basic interference assumptions do not hold. In the above example of OFDM systems, information is often modulated onto sub-carriers at a transmitter using Quadrature Amplitude Modulation (QAM) techniques. Conventional QAM demapping or demodulation of information from received signals in the presence of coloured interference leads to increased block or bit error rates (BLER/BER). Resultant performance losses can be on the order of about a 7-9 dB carrier to interference (CIR) penalty for a particular BLER or BER error floor.
One example type of communication system that is particularly prone to coloured interference is multiple-cell and multiple-access OFDM communication systems, generally referred to as OFDMA systems, in which frequency hopping is used by neighbouring cells and the hopping patterns are not orthogonal. In such systems, it is possible that mobile stations in adjacent cells may hop to the same sub-carrier at the same time, resulting in a relatively high level of interference on that sub-carrier but not necessarily on other sub-carriers. If sufficient information about interference sources, mainly channel state information, is available, then interference can be cancelled. However, the cost of interference cancellation in terms of signal processing can be significant. In addition, for up-link transmissions from mobile stations to base stations, it is unlikely that each base station can know the behaviour of all the mobile stations in its neighbouring cells. Therefore, cancellation of such inter-cell interference is not a practical option.
One way to reduce inter-cell interference is by scheduling mobile stations in coordinated patterns for neighbouring cells. Combined with up-link power control, coordination of mobile stations can mitigate inter-cell interference to a certain degree, but introduces additional communication system requirements to provide for such control of mobile station operations across different cells.